Comparative embryology by von Baer. 19th-century illustration of cross-sections and longitudinal sections through the embryo of a chicken, showing its structure and development. The egg yolk, which nourishes the embryo, is seen at upper left, with the embryo developing on its outer edge as shown in a series of diagrams. German scientist Karl Ernst von Baer (1792-1876) studied the development of animal embryos and compared them (a science known as comparative embryology), giving insights into animal evolution. His work led to Baer's laws of embryology. This page is the first plate from von Baer's 'Uber Entwickelungsgeschichte der Thiere' (On the evolutionary history of animals, 1828).